Straighten edges not on axis
This is one way to align verities (or straighten edges ) according to two of them (without moving these two ) :


select these two points (in this case the ends )
create an edge between them
set the manipulator to Normal
set the pivot to Active Element
set selection to Edge
select all where the created edge is the active one
hit S then Shift+Y and again Shift+Y, after that type 0 and hit 
Enter
delete the edge you created